Falls due to a loss of balance are a primary cause of injury and death in the elderly, and a debilitating symptom of a wide range of neurological, musculoskeletal, and cognitive deficits. However, because many neural and musculoskeletal elements can contribute to balance impairments - or equivalently to compensatory strategies - it is often difficult for clinicians to evaluate the severity of balance impairments, or to identify their underlying causes. The objective of this project is to understand the principles governing the modulation of feedforward and feedback neuromechanical elements contributing to postural stability during standing balance control. We have chosen to study the neuromechanical elements that can be rapidly modulated or selected by the nervous system, within the time frame of one session of data collection. We define feedforward neuromechanical elements to be those that adjust the intrinsic mechanical stability of the musculoskeletal system in anticipation of a postural perturbation, such as postural configuration and postural muscle tone. We define feedback neuromechanical elements to be those that activate muscles reactively following postural perturbations, and include task-level feedback gains, muscle synergies, and spinal reflexes. We hypothesize that the feedforward and feedback neuromechanical elements are modulated to achieve implicit performance goals such as stability, maneuverability, energy minimization, or robustness to uncertainty. Differing performance goals could explain variations in movement observed across trials, across individuals, across contexts, or across motor deficits. We predict that various combinations of neuromechanical elements may produce qualitatively similar movements, and yet quantitatively different functional and energetic consequences. We will study interactions between feedforward and feedback neuromechanical elements for standing balance control in normal and neurologically-impaired cats in Aim 1; during short- and long- term postural adaptations in intact animals in Aim 3. In Aim 2, we will identify tradeoffs between functional and energetic costs and constraints that may drive postural adaptations in both health and disease using neuromechanical models of postural control. This proposal continues our development of a general scientific framework toward our long-term goal of understanding, diagnosing, and predicting optimal motor function in individuals with balance deficits, and motor impairments in general.